What was in between them before they bumped?
Imagine a 2-D universe. It's a sheet of paper. It's infinitely large. Cartoon characters can crawl on that sheet; but they don't know the meaning of "up" or "down." That's your Flatland.
Now imagine that there is a stack of such sheets. Sometimes they hang one over the other, sometimes they "bump" into each other. (They are all parallel to each other.)
Can you explain "what is between them" sheets, using only the terminology of Flatlanders? You and me would say "air" or whatever else represents the 3rd dimension. But those poor Flatlanders have no such concept.
If we bump the whole model up a notch, the 4th dimension can be visualized as a gradual change of things that may already exist in our 3-D world. For example, what will happen when a 4-D sphere crosses through our 3-D plane of existence? First, there is nothing. Then a tiny sphere appears from nowhere. Then it grows. Then it shrinks, down to a tiny point, and then vanishes. What we see are 3-D sections of a larger, 4-D object.
So if there are multiple Universes that are separated by one or more other dimensions then when those Universes intersect they become visible to each other - just like when two paper sheets cross, the drawings on them (points, actually) become visible to both sets of Flatlanders. Also a Flatlander then can cross from his plane of existence onto the other one because the fact that the surface is sharply bent in 3rd dimension is lost on him.